might be good to remind users to override rust to nightly

[lbrindze]

in ubuntu 14.04 after installing multirust and running the command multirust update nightly I ran into the following error:

 #[feature] may not be used on the stable release channel

which suggested I was still using stable. running multirust override nightly did the trick, may be worth throwing into your document for other newbie readers such as myself.

Thanks for making a really excellent tutorial by the way!

[phil-opp]

Thanks!